Isaac: The good (drought buster) and the bad ($1 billion)

11:20 a.m. Update: Isaac has strengthened to Category 1 hurricane with 75 mph winds as it approaches Gulf Coast. Some further strengthening is likely as the storm nears the northern Gulf coast.

ORIGINAL ENTRY: Good morning. Isaac is nearing the Louisiana coast so let’s get right to it.

THE GOOD

Isaac remains a strong tropical storm this morning with 70 mph winds but it has not yet reached hurricane status despite having more than a day over the Gulf to intensify.

Hurricane scientists cite two reasons for its lack of intensification into a hurricane: dry air in the middle levels of the atmosphere that has worked its way into the storm’s circulation, and a rather large field of high winds. With respect to this latter point, you can think of this as the old ice-skating trick, you twirl around faster if you draw your arms into your body — it’s just so with a hurricane, if its core contracts its maximum winds grow stronger.

In any case, it now appears that Isaac will come ashore as a modest hurricane late tonight or tomorrow in southeastern Louisiana. This sure beats a larger hurricane pushing a 10-foot or higher surge. According to the National Hurricane Center there is a 50 percent chance of a storm surge exceeding 6 feet at a few areas along the Louisiana coast.

The forecast models remain in good agreement about Isaac’s landfall, and there’s now pretty high confidence in the storm’s track over the next five days. Here’s a look at the official forecast track map.

Isaac track map as of 4 a.m. CT. (National Hurricane Center)

There’s uncertainty about where the storm will go once it moves inland, but it has the potential to provide some beneficial rain to drought-stricken parts of middle America.

There is some potential for Isaac to address a serious drought situation in Arkansas, Missouri, Iowa and Illinois if it maintains a more northward track once inland. I know first hand there are very, very dry regions in the Ozark mountains that would love to get 6 inches of rain from Isaac. If it stays more northwest it could affect Oklahoma.

So that’s the good.

THE BAD

While there are potentially good elements of the storm, it is nonetheless a tropical system bearing down on the northern Gulf of Mexico coast. There will be consequences.

To get an idea of the realistic worst-case scenario for Isaac, consider the following analysis, by iCat, of damage caused by storms of similar size and track. Using the projection of a weak Category 2 hurricane making landfall in southeastern Louisiana, the group identified 15 analog storms.

Here is the group’s analysis:

It is estimated that 10 of the 15 would cause less than $1B in damage if they were to occur in 2012, while the other 5 would cause between $1B and $5B. The most similar historic track appears to be Gustav from 2008, which made landfall as a category 2 hurricane (track highlighted in orange). While Gustav was stronger than Isaac is expected to become, its center tracked well to the west of Baton Rouge and especially New Orleans.

Isaac’s ultimate economic impact will be highly dependent on how closely it tracks to these population centers. The further west it tracks, the higher the likelihood it will fall within the lower range of the storms in this analysis, while the opposite is true the closer it tracks toward New Orleans and Baton Rouge. Regardless of the exact landfall location, tropical storm force winds will be felt across a very broad area due to Isaac’s large size. Tropical storm force winds currently extend outward up to 205 miles.

The 15 analog storms caused a normalized amount of $1.16 billion, which means that’s the amount of damage they’re estimated to have caused if they were to strike the coast today. So Isaac is, at worst, probably a $1 billion tropical storm or hurricane.

A primary concern is inland flooding along and near the coast. The latest rainfall projections show the possibility if the core of highest rainfall coming near or over New Orleans. Although the city has a good pumping system to push water out of below-sea-level areas, that’s still a lot of pressure on the pumps.